The present invention relates in general to granulation of ammonium sulfate, and in particular to a method of manufacturing granules of ammonium sulfate during which a starting solution of the ammonium sulfate is spray dried and the resulting finely divided solid ammonium sulfate is, without delay, granulated by means of a finely sprayed aqueous medium, and thereupon the granules are dried.
Mineral fertilizers are normally brought to market in the form of pellets of a substantially uniform size of their grain. Also it is desired that the various components of the fertilizer be applied onto the field in a well mixed condition. In most cases, such fertilizing mixtures are already prepared in advance by the suppliers. Frequently it is also desired that the individual components of such mixed fertilizers be produced separately during their manufacture, in order to facilitate during their subsequent treatment the mixture in any desired proportion.
Due to different physical qualities of the various components of the mixed fertilizers, problems may occur during transportation of such mixtures because separation of the constituents may occur as a result of vibrations of the conveying device. The cause of such separations may be either the size of the grains or the shape of the grains. The mixed fertilizing product therefore tends to reach the consumer in a separated condition, and consequently a uniform application of the fertilizer onto the ground is no longer achievable.
As mentioned before, the decomposition problems occur particularly in the case when not only the grain size but also the grain configuration of respective components is different. For example, ammonium sulfate is customarily delivered in a crystallized condition. These crystals are predominantly substantially smaller than the grains of pelletized fertilizers. For this reason, the mixing of ammonium sulfate crystals with other fertilizing components is particularly susceptible to the aforementioned problems.
In addition, the crystallization of ammonium sulfate is a relatively difficult process, and accordingly the production of large crystals of ammonium sulfate is very expensive. Moreover, even if large crystals of ammonium sulfate are produced, there still would remain a substantial difference in the shape of the crystals relative to the grains of the pelletized fertilizing agent, and the danger of decomposition would still be present.
In view of these difficulties, prior-art methods described, for example, in the U.S. Pat. No. 4,305,748, assigned to the same assignee, has already achieved conversion of spray dried ammonium sulfate in crystalline form into pellets or granules. For this purpose, about 6-8% water (relative to the dry substance) has been finely sprayed on the spray-dried ammonium sulfate present on a pelletizing plate. During this process very uniform, almost spherical granules were produced which, already in the moist condition, proved to be very strong. Subsequent drying of these pellets yielded a product which was suitable for treatment in conventional fertilizing devices. This known fertilizer is sufficiently abrasion-resistant and, by suitably adjusting the pelletizing process, it can be manufactured in a wide variety of individual grain sizes.
Originally, the cause of this surprising effect of the prior-art method was unknown. Only after experiments were made to pelletize in this manner products from preceding manufacturing periods, was it recognized that a product stored for a prolonged period of time is extremely difficult to pelletize, and that the strength of pellets produced from such an old material is substantially lower in comparison with pellets produced from a fresh starting material. Further investigation of the properties of starting materials has shown that the aged starting material exhibits a distinct enlargement of crystals. Accordingly, it was recognized that crystalline ammonium sulfate is hardly suitable for pelletization.
This conclusion is understandable when one considers the fact that smooth large-area crystals of ammonium sulfate are probably subject to smaller adhesive forces than the substantially spherical, porous agglomerates composed of microcrystals only as normally grown during the spray drying.
During tests with manufacturing of pellets, another surprising effect of the drying process was found. In experiments directed to find a minimum residual moisture the pellets were occasionally subjected to a thermal treatment which substantially exceeded the time period normally required for the drying process itself. It was found that, depending on the duration of this thermal aftertreatment, an increased strength of the manufactured pellets was achieved.
In summary, by this known method according to the U.S. Pat. No. 4,305,748, the produced granulates possess a very high pressure- and abrasion-resistance, which nonetheless is still capable of a further improvement.